The present invention relates to a circuit for starting and operating a gas discharge lamp. More particularly, the present invention relates to an improved circuit for starting a gas discharge lamp regardless of the lamp temperature and for operating the lamp on DC voltage over a wide range of input voltage.
Gas discharge lamps, such as mercury vapor lamps, have a wide variety of uses, e.g., graphic arts plate development, printed circuit development, paint and epoxy curing, street lighting and as an ultraviolet source for fiber optics. While such lamps have many advantages, they have the drawback of being extremely difficult to restart when extinguished, even momentarily, after having been warmed up, since the relatively high vapor pressure in the hot lamp necessitates extremely high starting voltages. Thus, normally the lamp must be allowed to cool down for several minutes before restarting, which outage may constitute a severe drawback in certain applications. When started cold such lamps typically require several minutes to warm up to normal operating temperature and full light output.
Circuits have been provided for the rapid restarting of gas discharge lamps. One such circuit is disclosed in U.S. Pat. No. 3,944,876, issued to James G. Helmuth on Mar. 16, 1976, and in U.S. Pat. No. 3,732,460, issued to H. L. Wattenbach on May 8, 1973. But both of these prior devices are for use where AC voltage is applied to the lamp terminals, which tend to be less efficient than DC operated lamps, normally requiring higher operating voltages. Such circuits may utilize transient starting voltages of about 15,000 volts or higher, which present serious safety problems. Also, these devices are susceptible to the often substantial variations in AC line voltage.
Other starting circuits have been provided for use with lamps having DC operating voltage applied to the terminals thereof, such circuits being disclosed, for example, in U.S. Pat. No. 3,483,428, issued to D. E. LaPlante on Dec. 9, 1969, and U.S. Pat. No. 3,767,970, issued to J. R. Collins on Oct. 23, 1973. Both of these prior devices, as well as the starting circuits for AC-operated lamps discussed above, utilize transformer coupling of a high voltage transient starting pulse to one of the lamp electrodes simultaneously with the application of the operating voltage. Furthermore, these devices also fail to provide regulation of the power supply output over variations in input AC line voltage, and the starting circuits for DC-operated lamps are quite complicated.